This invention relates to a method for chemical vapor deposition of a layer of tungsten (W) on a semiconductor substrate.
The chemical vapor deposition of tungsten on a semiconductor substrate, such as a silicon oxide wafer which may have portions of an integrated circuit structure already formed therein, such as, for example one or more transistors, is an integral part of most semiconductor fabrication processes.
An insulating layer, mostly a silicon oxide layer has usually been formed over this substrate and has been previously patterned to provide openings or vias to underlying portions of the integrated circuit structure.
Chemical vapor deposited W has been used as a conducting material to fill contact holes or via holes. The tungsten layer covers the complete substrate surface and is then etched or polished away, except from the holes.
Since a tungsten layer cannot be deposited by chemical vapor deposition directly on a silicon oxide layer, an intermediate layer with a good adhesion for both the insulating layer and tungsten, for instance a titanium nitride (TiN) layer on top of titanium is deposited.
The tungsten is usually deposited through the reduction of tungsten hexafluoride (WF6) in a two-steps process. The steps are different in pressure set points and used reductor, being in the first step mainly silane (SiH4) and then hydrogen (H2) only. The largest part of the film is deposited by H2 reduction.
U.S. Pat. No. 5,028,565 of APPLIED MATERIALS, Inc., Santa Clara, Calif., LISA, discloses such method wherein tungsten is deposited on a wafer heated from about 350 to about 525xc2x0 C. in a vacuum chamber wherein the pressure is maintained from 2.67 to 101.32 kPa (from about 20 to about 760 Torr). A combination is used of WF6 gas, an inert carrier gas such as Ar, nitrogen and hydrogen. The flow rate of WF6 is from about 20 to about 200 standard cubic centimeters per minute (hereafter abbreviated as sccm). The flow rate of the inert carrier Ar is from about 100 to about 5000 sccm, and the flow rate of nitrogen is from about 10 to about 300 sccm. The hydrogen flow rate is from about 300 to about 3000 sccm.
The N2 in the gas mixture has been found to increase the reflectivity of the deposited layer which facilitates the use of photolitography in a subsequent patterning step, and to decrease the surface roughness.
U.S. Pat. No. 5,028,565 discloses however also that, especially when the intermediate layer is titanium nitride, it is important to form first a nucleation layer with from about 5 to about 50 sccm of WF6, from about 5 to about 50 sccm silane (SiH4), from about 500 to about 3000 sccm of Ar and from about 20 to about 300 sccm of N2.
It has been found that, without the nucleation layer, the tungsten layer was not uniform in thickness and resistivity.
Literature unanimously confirms the impossibility to obtain a tungsten film with good qualities, especially a good step coverage, a good layer uniformity and a low via resistance, without these two steps. The step coverage is the ratio of the thickness of the tungsten film at the side wall at half depth of the trench or contact hole and the nominal tungsten film thickness or the thickness of top layer.
EUI SONG KIM et al. for instance mention in their article xe2x80x9cStudies on the nucleation and growth of chemical-vapor-deposited W on TiN substratesxe2x80x9d, published in MATERIALS SCIENCE AND ENGINEERING, B 17 (1993) 137-142, that since it is not easy to nucleate W on TiN by H2 reduction of WF6, it is now common to initiate nucleation of W by SiH4 reduction first and then grow W film to the required thickness by H2 reduction.
CAROL M. McCONICA et al., also mention in their article xe2x80x9cStep coverage prediction during blanket LPCVD tungsten deposition from hydrogen, silane and tungsten hexafluoridexe2x80x9d, published in the Proceedings of the V-Mic Conference of Jun. 13-14, 1988, pages 268-276, Session VII: xe2x80x9cVSSI Multilevel Interconnection Dielectric Systemsxe2x80x9d, that the reduction with SiH4 or a mixture of SiH4 and H2 offers many advantages over the reduction by H2 alone, such as smaller temperature dependency in the growth rate, more uniform films and a larger growth rate, but that the major disadvantage of SiH4 is the limited step coverage, in comparison to the hydrogen reduction.
A. HASPER et al. In xe2x80x9cW-LPCVD step coverage and modeling in trenches and contact holesxe2x80x9d, Proceedings of the workshop on tungsten and other refractory metals for VLSI/USII applications V, 127 (1990) S. S. WONG and S. FURUKAWA ed., Materials Research Society, Pittsburg Pa., USA, mention also that the reduction with SiH4 offers many advantages like a high and temperature independent growth rate, a smaller grain size and has less interaction with silicon, but also that, when SiH4 is added to a WF6/H2 mixture, the step coverage drops.
In general, the hydrogen reduction gives better step coverage than the silane reduction, but the deposition rate of the hydrogen reduction method is significantly lower. Consequently, the second step in the tungsten deposition is therefore without SiH4 as in the actual method recommended by the above mentioned company APPLIED MATERIALS, INC.
This method comprises a soak step with SiH4, to saturate and passivate the underlying layer, a nucleation step at a pressure of 4.00 kPa (30 Torr), wherein 30 sccm WF6 is reduced by means of a mixture of 1000 sccm H2 and SiH4 in a flow ratio WF6/SiH4 of 2, and a bulk deposition step at a second pressure of 12.00 kPa (90 Torr) wherein sccm WF6 is reduced by means of 700 sccm H2 alone. The wafer is heated to 475xc2x0 C. during the tungsten deposition. An extra pressurizing step is necessary between both steps as there is a difference in pressure.
A similar method, but with both steps under the same pressure, is disclosed in U.S. Pat. No. 5,795,824 of NOVELLUS SYSTEMS, INC., San Jose, USA. After an initiation step by providing 15 to 75 sccm SiH4 and 1000 sccm Ar, the tungsten deposition is carried out under a pressure from 5.33 to 10.67 kPa (40-80 Torr) during successively two deposition steps: a nucleation deposition by providing from 1000 to 15000 sccm H2, from 50 to 800 sccm WF6 and from 15 to 75 sccm SiH4 and, in a different station, a bulk deposition by providing WF6, H2 and Ar gases, possibly in successive layers until the final thickness of tungsten.
All the above mentioned known methods with a reduction of tungsten hexafluoride in two steps are rather complicated and relatively slow, while a relatively complicated deposition system is required.
An object of the invention is to provide a method for tungsten chemical vapor deposition which is more simple and cheaper and has a higher deposition rate than the above mentioned prior art methods while a more simple deposition system may be used, and whereby the characteristics of the tungsten film such as the step coverage, the via resistance, the reflectivity etc. are at least equal to or better than these of a film obtained via the prior art methods.
According to the invention, this object is accomplished in a method for tungsten chemical vapor deposition on a semiconductor substrate, comprising positioning said substrate within a deposition chamber, heating said substrate and depositing under low pressure the tungsten on the substrate by contacting the latter with a mixture of gases flowing through the deposition chamber comprising tungsten hexafluoride (WF6), hydrogen (H2) and at least one carrier gas, characterized in that the mixture of gases comprises also silane (SiH4) with such a flow rate that the flow ratio WF6/SiH4 is from 2.5 to 6, the flow rate of WF6 being from 30 to 60 sccm, while the pressure in the deposition chamber is maintained from 0.13 to 5.33 kPa (I and 40 Torr).
It is amazing that by adjusting the flow ratio of WF6/SiH4, within the indicated pressure window, a 100% step coverage can be obtained.
Therefore, the tungsten deposition may be carried out in a single step.
Reaction efficiency is high, what results in high deposition rate and low gas consumption. Also the gas cost is low. There are less toxic gases and the overall quality of the tungsten film may be improved with respect to the prior art two step methods.
During the tungsten deposition, hydrogen is preferably supplied with a flow rate of 500 to 2000 sccm.
The temperature to which the substrate is heated depends amongst others on the chamber but is preferably situated between 400 and 495xc2x0 C., but may be extended to lower temperatures, what however results in a lower deposition rate.
Carrier gases may be Ar and N2 as in the prior art methods.